1. Field of the Invention
The present invention relates to a magnetic tape apparatus, and more particularly to a magnetic tape transport control unit in a reel-to-reel magnetic tape apparatus in which a tape is directly transported between reels without a mechanical buffer or a vacuum column buffer.
2. Description of the Prior Art
As a recording unit for recording and reproducing information, a magnetic tape apparatus which transports a magnetic tape by a magnetic tape transport mechanism having a mechanical buffer or a vacuum column buffer is frequently used. However, in the magnetic tape apparatus having such a magnetic tape transport unit, a cost is high because of the buffer unit, an entire unit is large in size and a noise is high. In order to resolve those problems, a reel-to-reel magnetic tape apparatus which has no buffer and transports the magnetic tape directly from a supply reel to a take-up reel is offered. In this apparatus, an entire unit is small in size because of lack of the buffer and a noise problem is also resolved. On the other hand, since a radius and a moment of a magnetic tape roll wound on each of the reels varies as the magnetic tape is fed from the supply reel, it is necessary to control the transport of the magnetic tape such that the magnetic tape is transported from one reel to the other under a constant tensional force and a constant speed.
To this end, for example, a tachometer is coupled to each of the supply reel and the take-up reel, or a tachometer having a fine scale is coupled to a tape transport path between the reels, and the changes of the radii and the moments of the tape rolls are determined as the tape is transported in order to maintain the tape tension and the tape speed constant. This method, however, operates well when a tape acceleration is low and the tape is started and stopped relatively slowly, but when the tape acceleration increases and the tape is transported at a high speed, a slip of the tape occurs at the tachometer mounted in the tape transport path and the tape tension and the tape speed cannot be precisely controlled.
In another approach, a tachometer is coupled to each of the supply reel and the take-up reel. One of the tachometers produces a pulse for each relatively large predetermined rotation angle while the other produces a pulse for each predetermined rotation angle which is smaller than the above predetermined rotation angle. Based on signals from those tachometers, a radius of tape roll on one of the reels is determined, and a radius of tape roll on the other reel is determined based on the determined radius of the one tape roll and precalculated constants relating to an actual thickness and a total length of the tape, and a tape speed, a tape acceleration and a tape tension are determined based on the determined radii and a reference tape speed to maintain the tape tension and the tape speed constant during the operation. In this method, however, the radii of the tape rolls on the respective reels can be exactly detected when the tape tension between the reels is constant but they cannot be exactly detected when the tape slacken. As a result, an overall performance is lowered because of a tape roll radius error. Further, when the tape tension between the reels is not stable and the tape speed changes, an error in the detected tape speed increases.
On the other hand, in recent years, a high density recording and a high speed recording of information have been demanded. In order to satisfy the demand, it is necessary to solve the above problems and control the magnetic tape tension and speed. However, a control method which satisfies the requirements has not been known so far.